This proposal involves the molecular genetic analysis of the mechanism of action, specificity, and control of bacteriophage P22 antirepressor. This novel regulatory protein represents a higher order of complexity in mechanisms of regulation of gene expression, since it works by interacting with another regulatory protein, phage repressor. Synthesis of antirepressor is in turn under negative control by an unknown mechanism. To determine the mechanism of action of antirepressor, I plan to purify the protein using an in vitro assay for inhibition of repressor activity. The interaction between antirepressor and repressor will then be studied in a defined system. The interaction between antirepressor and repressor can serve as a model for specific protein-protein interactions. A genetic approach will be taken to define the determinants of specificity of this interaction by isolating mutations that affect it. This study will also serve as a demonstration of the idea that a mutational alteration in one protein can correct or suppress a mutational alteration in a protein with which it interacts. A genetic approach will also be used to study the mechanism by which antirepressor synthesis is controlled. Mutants defective in control of antirepressor synthesis will be isolated and characterized in order to determine the number of gene products involved, their relationship, and their mechanism of action.